Correlation of metabolism with tissue carbon and nitrogen turnover rate in small mammals

Department of Biology, American University Washington D.C., Washington, Washington, D.C., United States
Oecologia (Impact Factor: 3.09). 12/2006; 150(2):190-201. DOI: 10.1007/s00442-006-0522-0
Source: PubMed


Stable isotopes have proven to be a useful tool for deciphering food webs, examining migration patterns and determining nutrient resource allocation. In order to increase the descriptive power of isotopes, an increasing number of studies are using them to model tissue turnover. However, these studies have, mostly by necessity, been largely limited to laboratory experiments and the demand for an easier method of estimating tissue turnover in the field for a large variety of organisms remains. In this study, we have determined the turnover rate of blood in mice and rats using stable isotope analysis, and compared these rates to the metabolic rates of the animals. Rats (Rattus norvegicus) (n=4) and mice (Mus musculus) (n=4) were switched between isotopically distinct diets, and the rate of change of δ13C and δ15N in whole blood was determined. Basal metabolic rates (as CO2 output and O2 consumption per unit time, normalized for mass) were determined for the rats and mice. Rats, which were an order of magnitude larger and had a slower metabolic rate per unit mass than mice (0.02 vs. 0.14 O2/min/g), had a slower blood turnover than mice for 13C (t
1/2 =24.8 and 17.3 days, respectively) and 15N (t
1/2 =27.7 and 15.4 days, respectively). A positive correlation between metabolic rate and blood isotopic turnover rate was found. These are the only such data for mammals available, but the literature for birds shows that mass and whole-body metabolic rates in birds scale logarithmically with tissue turnover. Interestingly, the mammalian data graph separately from the bird data on a turnover versus metabolic rate plot. Both mice and rat tissue in this study exhibited a slower turnover rate compared to metabolic rate than for birds. These data suggest that metabolic rate may be used to estimate tissue turnover rate when working with organisms in the field, but that a different relationship between tissue turnover and metabolism may exist for different classes of organisms.

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    • "In addition, we emphasize that laboratory conditions for feeding experiments are different from those in natural settings. Differences in environmental conditions (i.e., water temperature, diet) and biological factors (i.e., age, size, type of tissues examined) may ultimately affect estimates of metabolic rates and Hg isotope turnover rates in PBFT (Tieszen et al., 1983; MacAvoy et al., 2006). We suggest that these factors should be taken into consideration when applying Hg isotope measurements in fish to trace sources and changes in MMHg under various environmental conditions and ecosystem types. "

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    • "The growth rate constant, k, was estimated by fitting an exponential growth model to observed weight data, k = log(final weight / initial weight) / time, while parameter m was obtained using iterative non-linear regression. Coefficients k and m provide an indicator of the time period necessary for half of the muscle nitrogen to be replaced by new nitrogen after animals consume a new diet (half life, t 50 ) (MacAvoy et al., 2006). t 50 ¼ In2=m þ k ð2Þ where In2 represents the natural logarithm of 2. "
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    • "While it is essential to evaluate stable isotope dynamics across diverse species, there is a taxonomic bias in research on TDFs and isotopic incorporation rates within the vertebrates. Most studies have focused on fish, birds, and mammals (fish: Bosley et al. 2002; Logan et al. 2006; Suring and Wing 2009; Carleton and Martínez del Rio 2010; Hussey et al. 2010; Logan and Lutcavage 2010; Nelson et al. 2011; Kim et al. 2012; Heady and Moore 2013; birds: Hobson and Clark 1992a, 1992b; Bearhop et al. 2002; Ogden et al. 2004; Cherel et al. 2005; Hobson and Yohannes 2007; Bauchinger and McWilliams 2009; Connan et al. 2014; mammals: Tieszen et al. 1983; Roth and Hobson 2000; Lesage et al. 2002; MacAvoy et al. 2006; Stegall et al. 2008; Florin et al. 2011; Browning et al. 2014). Recent studies have also investigated TDFs and incorporation rates in reptiles (Seminoff et al. 2007, 2009; Reich et al. 2008; Fisk et al. 2009; Warne et al. 2010; Murrary and Wolf 2013). "
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